Auxiliary marine vessel propulsion system

ABSTRACT

The main propulsion and drive system of a merchant vessel such as a freighter or tank vessel, or a military vessel is supplemented by an auxiliary propulsion and drive system that includes at least one, but preferably a pair of auxiliary propellers that flank the main propeller at port and starboard positions and each of which are attached via a drive shaft through a gear train, which can optionally include a clutch mechanism, to an auxiliary motor or motors. In order to reduce the drag when the main propeller is disabled, segments of a wake field modifying propeller duct are mounted for repositioning to form at least a partial housing or cowling that diverts the water over the exterior surface of the duct and minimizes contact with the stationary blades of the main propeller as the vessel moves through the sea when powered by the auxiliary propulsion system.

RELATED APPLICATIONS

Not applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to vessel propulsion systems, and more particularly to an auxiliary vessel propulsion system employing one or more controllable pitch propellers.

2. Description of Related Art

Propellers have been the primary mode of propulsion for merchant marine and military vessels since they replaced paddle-wheel and wind-driven sailing vessels in the early 20^(th) century. While the scale, materials of construction, shape and engines have developed substantially over the past 100 years, the configuration of choice has essentially remained constant—single engine/single propeller design.

The tremendous scale and complexity of primary drive engines requires scheduled and unscheduled maintenance for optimum operation. For proper service, the vessel should be anchored or drifting. Additionally, engine failure is a problem which, while not necessarily very common, can yield disastrous consequences. For instance, in January 2007, the Vindu cargo vessel experienced engine failure, and came dangerously close to colliding with gas platforms and pipelines in the North Sea. Even when imminent danger is not a concern, the economic impact caused by a vessel that is anchored or adrift can be substantial, and cargo delays can cost resellers and/or sellers substantial sums of money in penalties, damaged business relationships, or result in useless goods such as perishables or seasonal merchandise.

Supertankers and container vessels have propulsion engines that can produce over 100,000 horsepower to turn the vessel's propeller and move the vessel through the water. This primary drive engine and other engines for driving electrical generators to provide power for the vessel's electrical systems are located in an engine room. While there is convenience in a single engine room for routine service and maintenance, in the event of damage, fire, attack or other disaster impacting the engine room, the vessel can be halted or set adrift.

Different types of auxiliary propellers on marine vessels have been described in the patent literature. For instance, U.S. Pat. No. 1,577,101 describes a system having a main propulsion propeller and two auxiliary propellers used as generators. The generators can be disabled by means of clutches between auxiliary shafts. However, when the vessel is operated by the main propulsion propeller, the turning of the auxiliary propellers creates hydrodynamic drag, lowering the efficiency of the propulsion system. In addition, the main propeller and the auxiliary propellers are operably interrelated, so the auxiliary propellers are essentially rendered useless in the event that the main propeller system fails.

U.S. Pat. No. 3,993,912 discloses a marine propulsion system that includes a gas turbine, a generator, a three-phase AC electric motor and a fixed pitch propeller. In the normal ahead and astern directions, the motor operates synchronously; in the slow ahead and astern directions, the motor is operated asynchronously with the gas turbine operating at its idle setting. Means are also provided to break the propeller before astern torque is applied when the power propulsion lever is rapidly moved from the normal ahead setting to the normal astern setting by operating the motor as a generator. However, there is no consideration given to propelling the vessel when the motor and/or the gas turbine fail or requires maintenance.

U.S. Pat. No. 4,668,197 discloses a retractable auxiliary and emergency propulsion device for small craft. The device includes a hydraulic motor with its own propeller powered by a hydraulic pump, coupled with either a dedicated motor installed on the craft or existing motors on the boat for purposes other than propulsion. The hydraulic motor slides on inclined guides located in the stern of the craft so as to be dropped into the sea when necessary, or raised up and placed in a suitable storage space in the boat. However, this device is intended for small watercraft, and is not suitable as a backup propulsion system for large merchant vessels.

Therefore, a need exists for a reliable, fail-safe system for minimizing vessel delays and loss of control in merchant and military vessels during a period of time that the main propulsion drive system fails or is shut down for scheduled or unscheduled maintenance at sea.

Accordingly, it is an object of the present invention to provide an auxiliary propulsion system for vessels that does not interfere with the routine operation of the main propulsion and drive system.

It is another object of the present invention to provide a system for auxiliary propulsion that optionally provides the additional benefit of power generation.

SUMMARY OF THE INVENTION

The above objects and further advantages are provided by the apparatus, system and method of the invention in which the main propulsion and drive system of a merchant vessel such as a freighter or tank vessel, or a military vessel is supplemented by an auxiliary propulsion and drive system that includes at least one, but preferably a pair of propellers which flank the main propeller and which are attached via a drive shaft, through a gear train, which can optionally include a clutch mechanism, to an auxiliary motor or motors. It will be understood by one of ordinary skill in the art that use of a single auxiliary propeller, positioned either port or starboard of the main propeller will require the rudder to be turned in order to maintain a straight ahead course. This positioning of the rudder creates a certain amount of hydrodynamic drag. Use of two auxiliary propellers eliminates this use of the rudder.

In the following description, when reference is made to a propeller in the singular form, it will be understood that the description is equally applicable to the preferred embodiment in which two auxiliary propellers are utilized.

In the interest of simplicity and clarity, the invention will be described with reference to a vessel having a single main drive engine and, in any event, it will be assumed that the vessel is without an operable main propeller.

In a preferred embodiment, when no power is being applied to the auxiliary propeller(s), the pitch of the blades are adjustable to one or more predetermined positions, one of which positions reduces the hydrodynamic drag, and therefore the tendency to cause the auxiliary propeller to rotate as the vessel is propelled through the sea by the main propeller. This will be referred to as feathering the propeller, or to a feathered propeller.

As used herein, the term “feathering” includes any movement of the propeller blades that has the effect of reducing the propeller's hydrodynamic drag on the vessel as it moves through the water.

In another particularly preferred embodiment, the pitch of the blades of the auxiliary propeller can be adjusted to cause the auxiliary propeller to rotate the drive shaft at a rate that is optimum to turn an electrical generator located aboard the vessel which can be used to power vessel systems and/or to charge storage batteries.

When the auxiliary propellers are not being used to operate the electrical generator(s), the blades are preferably feathered and the propeller drive shaft is disengaged via a clutch or other gear train mechanism so that the propeller is free-wheeling with the minimum number of on-board components in the drive train moving in order to minimize frictional drag forces in bearings and the like.

As will be understood by one of ordinary skill in the art, the size of the auxiliary motor(s) must be sufficient to drive the auxiliary propellers at a speed that will produce a propelling force sufficient to provide headway that will allow the vessel controls, which may be limited to the rudder, and perhaps bow thrusters, to direct the movement of the vessel and prevent it from being carried into a broaching or other hazardous position as a result of the action of currents and/or waves, or the vessel's own momentum. Although the determination of the power output, e.g., horsepower of the auxiliary motor(s) is readily determined, such calculations must take into consideration the vessel's maximum load capacity, as well as the worse case scenario for adverse conditions of the weather and seas through which the vessel can be expected to ply. For example, if a vessel's main engine is capable of generating up to 100,000 horsepower which enables it to safely navigate based upon its size, maximum load and foreseeable weather and sea conditions, it may be possible to control the vessel in order to maintain a safe course using an auxiliary power source of as little as 2,000 horsepower. A reasonable design parameter is presently believed to be an auxiliary motor that has from about 5% to about 15% of the horsepower of the main engine.

A pair of propellers flanking the main propeller can be powered by a single auxiliary motor with an appropriate gear train and drive shafts in accordance with any of a number of mechanical arrangements that will be readily apparent to one of ordinary skill in the art. In addition, the speed of each of the auxiliary propellers can also be controlled independently by means of an automatic power transmission device associated with each drive shaft, the transmission or gear boxes being located in the respective drive trains. Thus, by varying the relative speed of the auxiliary propellers, the course of the vessel may be directed, at least to some extent, and the rudder can be used to a lesser extent.

Assuming that each auxiliary propeller is powered by a separate auxiliary motor, the speed of each of the auxiliary propellers can be controlled by adjusting the speed of the motors independently and/or the use of automatic transmission devices or adjustable gear boxes. Likewise, the direction of rotation of each of the auxiliary propellers can be reversed in order to facilitate a more rapid change in course or to assist in docking the vessel.

As will be apparent to one of ordinary skill in the art, only a single auxiliary propeller is mounted to the port or starboard side of the main propeller, the energy required to maintain the vessel on course and making sufficient headway will have to be increased due to the drag effect of controlling the forward direction using the vessel's rudder.

In order to maximize the efficient operation of a vessel at sea that is relying solely on the auxiliary propellers, in a preferred embodiment of the invention, the main propeller is disengaged from the drive shaft and any gear train stemming to the main engine. In a particularly preferred embodiment, the pitch of the blades of the main propeller is adjustable to a position that minimizes the hydrodynamic drag of the water moving across the blades.

In order to reduce the drag when the main propeller is disabled, in a particularly preferred embodiment, a series of efficiency fins mounted forward of a fitted propeller duct are rotated 90°, and preferably folded in the aft direction, to form at least a partial housing or cowling that diverts the water over the duct and out of contact with the stationary blades of the main propeller.

In another preferred embodiment, a duct or vane having movable segments defining a water channel is installed between the vessel's hull and the forward portion of the main propeller and surrounds the main propeller drive shaft, and is maintained in a first open position while the main engine is used to propel the vessel. In the event that the main propeller stops while the vessel is underway, the moveable segments forming the walls of the vane or duct are reoriented and moved toward each other to thereby form a cone having its larger open end in proximity to the stationary main propeller blades and the forward end proximate the main drive shaft, thereby reducing the drag force that would otherwise be created by water contacting the surfaces of the stationary propeller. The cone is of a truncated configuration and can be formed from two opposing curvilinear sections that in the open position serve to channel water passing from the vessel's hull toward the propeller into a more efficient slip stream. Alternatively, the cone can be formed from more than two segments, each of which segments can be of a flat or curvilinear configuration. The movement of the cone segments can be controlled by hydraulic supporting struts or rams and/or other mechanisms known in the field of marine engineering and architecture.

In one embodiment, the auxiliary propulsion and drive system of the invention is only intended to be activated during what would be considered emergency conditions when the main propeller has ceased to operate while the vessel was underway at sea or in other port locations where one of more tugboats or other similar vessels cannot provide immediate assistance to prevent damage to the vessel and/or the surrounding facilities.

It should also be understood that the orientation of the blades of the propeller can be adjusted relative to the drive shaft in order to attain more efficient operation, for example, to increase power when initiating movement from a stopped position, i.e., high torque and lower RPM, or to increase speed, i.e., low torque and relatively higher RPM when the vessel is underway.

It is known in the art to install a generally tubular stationary duct between the propeller and the vessel's stern to modify the wake field of full-form vessels such as tankers in order to improve fuel efficiency. In accordance with a further embodiment of the invention, a two-part moveable duct is mounted co-axially with the main propeller drive shaft between the propeller and the stern to provide in a first position, an open body formed of curvilinear segments to modify the wake form when the vessel is being powered by the main propeller, and in a second position, a truncated conical surface, with the smaller end forward towards the hull and relatively closer to the drive shaft, and the aft end defining an opening adjacent to the path circumscribed by the rotation of the ends of the main propeller blades. In the second position the conical surface serves to deflect the water outwardly to thereby by-pass the stationary blades of the main propeller and reduce the hydrodynamic drag on the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail below and with reference to the attached drawings in which the same or similar elements are referred to by the same number, and where:

FIG. 1 is a schematic diagram of a propulsion system according to the present invention;

FIGS. 2A and 2B are enlarged depictions showing in more detail the operational positions of a preferred embodiment of the apparatus of FIG. 1;

FIG. 3 is a view taken along line 3-3 of FIG. 1; and

FIG. 4 is a flow chart of an embodiment of the method of the invention.

In the interest of clarity and to facilitate the depiction of the inventive features, the vessel's rudder has been omitted from the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a propulsion system 10 for a merchant vessel or the like is schematically depicted, including a main propulsion and drive subsystem 12 and a pair of auxiliary propulsion drive subsystems 14 a and 14 b, collectively referred to as 14, that can be operated independently of the main propulsion and drive subsystem 12. Generally, the main propulsion and drive subsystem 12 propels the vessel under normal operating conditions in accordance with known and conventional practices. According to the present invention, if the main propulsion drive subsystem 12 fails, or when the main propulsion drive subsystem 12 must be shut down for routine or unscheduled maintenance while at sea, the auxiliary propulsion drive subsystem 14 is used to propel the vessel, thereby offering reliable back-up propulsion. As will be understood by one of ordinary skill in the art, in a preferred embodiment the pair of auxiliary systems 14 a and 14 b flank the main drive system 12 at corresponding port and starboard positions in order to provide a balanced power source, and also to facilitate navigation by independently controlling subsystems 14 a and 14 b, e.g. by running the two propellers in opposing rotational directions. However, the method and system of the present invention comprehends the use of a single auxiliary propulsion drive system which, in conjunction with the vessel's rudder, can be used to achieve the goals and benefits described.

More particularly, and still referring to FIG. 1, the main propulsion drive subsystem 12 includes a main engine 16 selectively coupled to a fixed propeller 18 via a driveshaft 17. The main engine 16 is connected to the fixed propeller 18 through a clutch system 20.

The auxiliary propulsion drive subsystem 14 includes an auxiliary motor 24 selectively coupled to an auxiliary controllable pitch propeller 26 via a driveshaft 25. The auxiliary motor 24 is connected to the auxiliary propeller 26 through an optional clutch system 28. In certain embodiments, the auxiliary propulsion drive subsystem 14 also includes a gear train and generator apparatus 30 intermediate the clutch system 28 and the controllable pitch propeller 26 for operating the auxiliary propulsion drive subsystem 14 in a power generation mode. In general, the auxiliary motor 24 has substantially less horsepower then the main engine 16. For example, where the main engine 16 is rated at from 30,000 to greater than 100,000 horsepower for a merchant vessel, the auxiliary motor can have a rating that is about 5% to about 15% of the horsepower of the main engine 16.

The auxiliary motor 24 can be an electric motor powered by an electrical system of the marine vessel, or powered by dedicated electrical storage batteries. In the latter configuration, the auxiliary propulsion drive subsystem 14 provides a fail-safe back-up or emergency drive power source, even if the vessel's entire main propulsion and drive subsystem 12 and its associated electrical system fails.

Alternatively, the auxiliary motor 24 can be an internal combustion engine. The internal combustion engine can be a diesel engine or a gasoline engine. Preferably, even if the type of fuel for the main engine 16 and the auxiliary motor 24 is the same, the fuel for the auxiliary motor 24 is stored separately and is isolated from the fuel for the main engine 16. Since most large vessels utilize bunker oil as fuel, the smaller auxiliary motor 24 can be more efficiently powered by diesel or gasoline.

In a propulsion mode of the main propulsion drive subsystem 12, the clutch system 20 of the main propulsion drive subsystem 12 engages the main driveshaft 17 to allow the engine 16 to turn the propeller 18, and the fixed vane cone 22 is in an open configuration to allow water to flow through and the rotation of the propeller 18 functions to move the vessel forward. While the main propulsion subsystem 12 is operating in its normal propulsion mode, the clutch system 28 is of the auxiliary propulsion drive subsystem 14 is disengaged from the auxiliary motor 24 thereby preventing rotation of the controllable pitch propeller 26 by the auxiliary motor 24.

In certain embodiments, the gear and generator apparatus 30 can be selectively engaged with the controllable pitch propeller 26. Accordingly, in the propulsion mode of the main propulsion drive subsystem 12, while the vessel is being driven through the water by power from the main engine 16, the blades of the trailing controllable pitch propeller 26 are pitched to maximize hydrodynamic forces to cause rotation of the propeller 26, and the rotational forces are transmitted through the driveshaft 25 to the gear and generator apparatus 30. The generator apparatus 30 can be electrically coupled to one or more electrical storage batteries, to one or more electrical loads on the vessel directly or through an electrical distribution system, or to both the storage batteries and the electrical loads. As is conventional in power generation apparatus, the electrical output from the generator can be coupled to the batteries and/or other loads through one or more suitable power conditioners, such as AC-DC inverters and/or voltage converters.

During a disengaged mode of operation of the main propulsion drive subsystem 12, e.g., corresponding to periods of engine failure or shut down for maintenance, the clutch system 20 and the driveshaft 17 are disengaged from the engine. In addition, the fixed vane cone 22 is moved to a closed configuration to divert water flow past the stationary propeller 18 and prevent its rotation while it is disengaged from the engine 16.

When the main propulsion drive subsystem 12 is disengaged, the auxiliary propulsion drive subsystem 14 can be activated to operate in the propulsion mode. The auxiliary controllable pitch propeller 26 is adjusted to a drive configuration to provide forward propulsion force to the vessel. In the propulsion mode of the auxiliary propulsion drive subsystem 14, the clutch system 28 is engaged to couple to the auxiliary motor 24 and rotate the controllable pitch propeller 26.

As will be apparent to one of ordinary skill in the art, a control system for switching to the auxiliary mode electronically can be provided using an appropriately programmed processor/controller that is responsive to an emergency shut-down of the main engine or unexpected failure. For example, the control system can be programmed to automatically initiate the change-over to the auxiliary power system, or to provide an alarm so that vessel's personnel can make the decision to manually initiate the action. The back-up or emergency control system can be connected to the storage batteries or emergency power generation system in the event that the main power fails

When operating in the propulsion mode, the auxiliary propeller 26 provides the propulsion force to allow the vessel to move through the water, even while the main engine 16 is undergoing repairs and/or maintenance. By disengaging the main engine from propeller 18 via the clutch system 20 as described above, and when the fixed vane cone 22 is in the closed position, the hydrodynamic drag forces on the main fixed blade propeller 18 is minimized as the vessel moves under power of the auxiliary propulsion drive subsystem 14.

If the main propeller has controllable pitch blades, the blades are adjusted to minimize the drag of the stationary blades. While the auxiliary propulsion drive subsystem 14 is not intended to provide horsepower that is comparable to the main propulsion drive subsystem 12, the engine(s) or motor(s) are selected to provide sufficient power to propel the vessel at a speed that will permit the use of the rudder to control the vessel's direction to avoid collisions, and make headway while the main engine 16 or other components of the main propulsion drive subsystem 12 are being repaired or serviced.

In order to adjust the pitch of the controllable pitch propeller, a suitable feathering apparatus (not shown) is also incorporated in the auxiliary propulsion drive subsystem 14. Controllable pitch propellers are well known in the art, as are the mechanisms for changing the pitch of the blades to two or more predetermined positions. These mechanical systems and their components form no part of the present invention.

For instance, in one embodiment, a mechanical subsystem for feathering the propeller is provided that moves the propeller blades to minimize the hydrodynamic drag forces when the vessel is moving under the main power system.

In another embodiment, a mechanical subsystem for feathering the propeller is provided that moves the blades of the propeller to a position in which the edges of the blades are in line with a direction of marine traverse during the disengaged mode of operation, and to a water-contacting position during the engaged mode of operation.

Also as shown in FIG. 1 generally, and in more detail in FIGS. 2A, 2B and 3, an optional, but preferred embodiment is shown in which a vane or duct 50 with moveable segments is installed axially about the driveshaft 17 between the vessels stern and the main propeller 18 to modify water flow patterns in the region of the main propeller.

With reference to the embodiment schematically illustrated in FIGS. 2A and 2B, the vessel is provided with a vane or duct 50 having moveable elements 50A and 50B. As shown in FIG. 2A, the elements are in the open position and assist in reducing the turbulence in the water following the stern of the hull, thereby improving the efficiency of the main propeller.

Referring now to FIG. 2B, the forward ends of segments 50A and 50B are moved toward the main drive shaft 17 to form a truncated conical surface. In this position, the duct 50 provides a streamlined surface in the form of a housing that directs the water outwardly to minimize direct impingement on the stationary blades of the main propeller when it is out of operation. Thus, when the vane or duct segments 50A and 50B are moved to the enclosing position shown in FIG. 2B, they form a surface that minimizes turbulence and produces a laminar flow, to the extent possible, as determined by limitations of spacing and configuration of the vessel's hull, location and range of movement of the vessel's rudder and overall spacial relations that will be apparent to one of ordinary skill in the art. During ordinary operation of the main propeller, the segments of the duct or vane are moved to stationary positions in which the individual segments themselves present minimum hydrodynamic drag and also serve to channel the water trailing the adjacent lower hull portion at the vessel's stern.

Referring to FIG. 3, a view aft from the hull position shows the segments 50A and 50B in their open configuration during normal propulsion of the vessel by the main propeller 18, with the auxiliary propellers 25P and 25S stationary, and preferably feathered to minimize their hydrodynamic drag on the vessel.

With continuing reference to FIGS. 2A, 2B and 3, a supporting member 52 is, illustratively, mounted around drive shaft 17 and includes a cross-member 53 to which the aft portions of segments 50A and 50B are mounted at pivot points 56. Other mechanical arrangements and details for supporting the segments comprising the duct 50 and the related elements for adjoining its position will be apparent to those of ordinary skill in the art.

As previously noted, the segments 50A and 50B in this open configuration modify the wake field of the water streaming from the vessel's hull at the stern end to reduce turbulence before the propeller.

This step-wise practice of the method of the invention will be further described with reference to the flowchart of FIG. 5. With the main engine stopped, 100, due to an emergency or predetermined maintenance requirement, the main propeller blades or optionally feather 102. If present, the wake field control duct is adjusted from its open position to its closed conical position 104.

If provided and operational, the electrical generator is disengaged from the auxiliary propeller(s) 110.

With the main engine stopped and the auxiliary propellers moved from an optional feathered position to the operational position 106, the auxiliary drive motor(s) are activated 120 and the drive shaft(s) for the auxiliary drive propeller(s) are engaged 122.

During continuing operation of the auxiliary propulsion system, the speed and direction of rotation of the auxiliary drive motor(s) are adjusted 124 to maintain the desired speed, and optionally the course of the vessel.

As previously noted, the above steps can be undertaken by the vessel's personnel, or essentially automatically in accordance with an appropriately programmed computer processor and controller.

It will also be understood from the above description, the steps identified as optional, e.g., 102, 104, 106 and 110, are designated as such because the particular feature may not be present on the vessel, because it was not engaged at the time that the main engine stopped. For example, retrofitting an existing vessel with a controllable-pitch main propeller for use with the auxiliary propulsion system of the invention may not be commercially practical.

In summary, the principal aspects of the preferred embodiments of the present invention comprise an auxiliary propulsion drive subsystem that includes a disengaged mode, a power generation mode and a propulsion mode, in which the propeller blades are adjustable to at least three corresponding pitches to provide the most efficient and effective operation. Additional pitch configurations can be provided to provide high torque/low RPM and low torque/high RPM operation of the auxiliary power system.

The method, apparatus and system of the present invention have been described in detail above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be defined by the claims that follow. 

1. A marine vessel propulsion system for a marine vessel comprising: a. a main propulsion drive subsystem that includes a main propeller and a main engine, the main propulsion drive subsystem including a propulsion mode in which the main propeller is operably connected to the main engine, and a disengaged mode in which the main propeller is disengaged from the main engine; and b. an auxiliary propulsion drive subsystem that includes at least one auxiliary propeller and at least one auxiliary motor, the auxiliary propulsion drive subsystem including a disengaged mode, coincident with the propulsion mode of the main propulsion drive subsystem, in which the auxiliary propeller is disengaged from the auxiliary motor; and a propulsion mode coincident with the disengaged mode of the main propulsion drive subsystem in which the least one auxiliary propeller is operably connected to at least one auxiliary motor.
 2. The marine vessel propulsion system of claim 1 in which the main propulsion drive subsystem includes a drive shaft in selective mechanical cooperation with the engine and with the propeller, and a clutch for engaging and disengaging the main engine from the propeller to effectuate selective mechanical cooperation to permit free rotation of the main propeller under the influence of applied hydrodynamic force whereby the overall hydrodynamic drag on the moving vessel is minimized in the disengaged mode of the main propulsion drive subsystem.
 3. The marine propulsion system of claim 1, wherein the auxiliary propulsion drive subsystem further includes a mechanical subsystem for feathering the at least one auxiliary propeller when the auxiliary propulsion drive subsystem is in the disengaged mode.
 4. The marine propulsion system of claim 3, wherein the mechanical subsystem for feathering the at least one propeller operates to turn the blades from a position that is generally transverse to the drive shaft to a position that is generally aligned with the drive shaft.
 5. The marine propulsion system of claim 1, wherein the auxiliary propulsion drive subsystem further includes a generator that is selectively coupled to the at least one auxiliary propeller for operation in a power-generating mode while the vessel is moving.
 6. The marine propulsion system of claim 1, wherein at least one motor of the auxiliary propulsion drive subsystem is an electric motor that is powered by the vessel's electrical generation system and/or electrical storage batteries.
 7. The marine propulsion system of claim 1 that includes two propellers that flank the main propeller, each of which auxiliary propellers is associated with an independently controlled auxiliary motor.
 8. The marine propulsion system of claim 2 that includes a wake field modifying apparatus comprising a two-part duct that is mounted coaxially with the main drive shaft and is movable from a first position in which opposing arcuate members define an open cylindrical form to a second position defining an open-ended truncated conical form.
 9. The system of claim 8 in which the opposing arcuate members are mounted for pivotal rotation about an axis that is transverse to the drive shaft.
 10. The system of claim 8 in which the aft end portions of the opposing arcuate members define a generally circular opening that has a diameter that is at least as great as the diameter of the main propeller.
 11. An auxiliary propulsion apparatus for a marine vessel having a main propeller mounted on a drive shaft that is operatively connected to the vessel's main engine, the auxiliary propeller apparatus comprising: a pair of auxiliary propellers flanking the main propeller, each auxiliary propeller being operatively linked to an auxiliary motor through a clutch mechanism operatively positioned between the main engine and the main propeller driveshaft, whereby the main propeller can be disengaged when the main engine is stopped.
 12. The apparatus of claim 11 in which the blades of the auxiliary propellers are moveable from one or more operable drive positions to a feathered position.
 13. The apparatus of claim 11 in which the blades of the main propeller are moveable from one or more drive positions to a feathered position.
 14. The apparatus of claim 11 which further includes an electrical generator that is configured to selectively engage the drive shaft of an auxiliary propeller.
 15. The apparatus of claim 11 in which each auxiliary propeller is linked to a separate motor.
 16. The apparatus of claim 11 which also includes a variable gear mechanism operatively attached to each of the auxiliary propeller drive shafts.
 17. The apparatus of claim 11 which includes a wake field modifying apparatus comprising a two-part duct that is mounted coaxially with the main drive shaft and is movable from a first position in which opposing arcuate members define an open cylindrical form to a second position defining an open-ended truncated conical form. 